Disk-wire mechanical filter using bridging wire to achieve attenuation pole

ABSTRACT

A general stopband disc-wire type mechanical filter having at least four circle mode vibration-type discs therein with a first coupling wire means connected to the perimeters of all four discs and a second coupling wire means connected only to the first and the fourth disc and bridging the two discs therebetween. The second bridging coupling wire means has a length such that it produces a 180* phase shift of energy transferred therethrough with in the passband. Circle mode-type discs resonate in phase with each other at the lower end of the passband and out of phase (with the adjacent discs) at the upper end of the passband. Consequently, the energy transfer through the bridging coupling wires is out of phase with the energy transfer through the first coupling wire means both at the lower and upper ends of the passband, thereby producing the general stopband characteristic.

United States Patent 3 [72] Inventor Roger J. Teske Santa Ana, Calif.[21] Appl. No. 834,978 {22] Filed June 20, 1969 [45] Patented Mar. 23,1971 [73] Assignee Collins Radio Company Cedar Rapids, Iowa [54]DISK-WIRE MECHANICAL FILTER USING BRIDGING WIRE TO ACHIEVE AT'IENUATIONPOLE Claims, Drawing Figs.

[52] US. Cl .1 333/71, 333/ [51] Int. Cl 03h 9/26 Field of Search333/30, 70, 71

[56] References Cited UNITED STATES PATENTS 2,856,588 10/1958 Bums333/71 2,918,634 12/1959 Bercovitz 333/71 3,135,933 6/1964 Johnson333/71 3,142,027 7/1964 Albsrneier et a1...... 333/71 3,351,875 11/1967Midgley 333/71 3,439,295 4/1969 Bise 333/ 72X 3,440,572 4/1969 Bise333/71X 3,440,574 4/1969 Johnson et a! 333/71X Primary Examiner-HermanKarl Saalbach Assistant ExaminerMarvin Nussbaum Att0rneys-Henry K.Woodward and Robert J. Crawford ABSTRACT: A general stopbanddisc-wiretype mechanical filter having at least four circle modevibration-type discs therein with a first coupling wire means connectedto the perimeters of all four discs and a second coupling wire meansconnected only to the first and the fourth disc and bridging the twodiscs therebetween. The second bridging coupling wire means has a lengthsuch that it produces a phase shift of PATENTEDmzs I97! $571,766

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mvsur on; ROGER a. resxs I BY ATTORNEY DISK-WERE MECHANKCAL FILTER USINGBlllll LGlNG Willi: TO AEHHEVE ATKENUATKQN FQLE This invention relatesgenerally to mechanical filters and, more specifically, to a generalstopband filter to a general stopband filter of the stackeddisc-coupling wire type.

Disc-wire type mechanical filters have been quite well developed in theart. These type filters generally comprise a plurality of discs stackedone upon the other with their axes lying along a common line and spacedapart usually not more than one wavelength of the nominal centerfrequency of the filter pmsband. Most of the disc-wire type mechanicalfilters employ a circle mode of vibration in the discs. The circlemode-type vibration can be analogized to the action of the bottom of anoil can in that the nodes are circular and concentric around the axis ofthe disc. However, some of the discwire type filters employ one or morediscs having a diameter mode of vibration. A diameter mode of vibrationis characterized by nodes which coincide with diameters of the disc.

in the absence of special construction features a disc-wire typemechanical filter employing circle mode discs does not have attenuationpoles either at the upper edge or the lower edge of the passband. Whilethe frequency response characteristic of such a mechanical filter isquite good, there are applications where it is desirable to have sharpercutoff characteristics. Such sharper cutoff characteristics can beeffected by attenuation poles at the upper and lower edges of thepassband.

One means for producing an attenuation pole at the upper end of thepassband is described in US. Pat. No. 3,135,933 issued .lun. 2, 1964, toRobert A. Johnson and entitles M- Derived Mechanical Filter." In thisstructure a coupling wire is caused to bridge a disc and to bephysically attached to the discs on either side of the bridged disc.Other coupling wire means are physically attached to all three discs.Since it is a characteristic of mechanical filters that each discvibrate 180 out of phase with the adjacent disc, at the upper edge ofthe passband, the energy transferred through the bridging coupling wirewill be 180 out of phase with the energy transferred through the bridgeddisc. Cancellation of the two signals occurs to produce the attenuationpole at the upper edge of the passband.

In US. application, Ser. No. 614,621, filed Feb. 8, 1967, now US. Pat.No. 3,488,608, by Robert A. Johnson and entitled General StopbandMechanical Disc Filter Section Employing MultiMode Disc" there isdescribed a structure for obtaining attenuation poles both at the upperand lower edges of the passband through the use of discs operating inthe diameter mode vibration. It is a characteristic of the diameter modevibration that one or more portions of the perimeter of the discs willbe vibrating in a given phase and the diametrically opposed portions ofthe discs will be vibrating in a phase 180 removed therefrom. Because ofthe coexistence of these two phases of vibrations in the same disc, itis possible to provide for cancellation of the signal both at the upperand the lower edges of the passband. Another structure employing thediameter mode-type disc which has attenuation poles both at the upperand lower edges of he passband is described in US. application Ser. No.557,300 filed Jun. 13, 1966, now US. Pat. No. 3,439,295, by Donald L.Bise and entitled Mechanical Filter With Attenuation Poles On Both SidesOf Passband."

Another related Pat. is Mechanical Filters Employed MultilviodeResonators, U.S. Pat. No. 3,516,029, by Robert A. Johnson.

While the aforementioned general stopband type mechanical filtersprovide excellent response characteristics, certain problems ofmanufacture are present. More specifically, coupling wires attached tothe perimeters of the diameter mode-type disc are quite sensitive to thedetailed structure of the connection to the disc. If the welding of suchcoupling wire to the disc is not performed within quite closetolerances, the attenuation pole is moved up or down the frequency scaleand thus can be moved either too far away from the edge of the passband,or conversely it can be moved into the edge of the passband, both ofwhich conditions are undesirable.

It is a primary object of the present invention to provide a reliablegeneral stopband mechanical filter which is relatively easy tomanufacture.

A second purpose of the invention is to provide a simple, study andreliable general stopband mechanical filter of the disc-wire type.

' A third purpose of the invention is to provide a general stopbandmechanical filter in which the general stopband characteristic isprovided by a unique arrangement of coupling wires rather than throughthe use of diameter modes discs.

A fourth object of the invention is to provide a means for obtainingattenuation poles at both the upper and lower ends of the passband of adisc-wire mechanical filter by the unique construction and arrangementof coupling wires.

A fifth aim of the invention is to provide a general stopband mechanicalfilter'employing all circle mode-type discs.

A sixth aim of the invention is the improvement, generally, ofmechanical filters of the disc-wire type.

in accordance with the invention there is provided a general stopbandmechanical filter comprising a plurality of circle mode-type discsstacked one upon the other with their axes lying along a common line. Afirst plurality of coupling wire means are positioned parallel to thecommon axis and along the perimeters of the discs, and secured theretoto hold said disc in position and also to transfer energy from disc todisc along the filter. Second coupling wire means, which form theessence of the present invention, couple together two discs which areseparated by two bridged discs. More specifically the second couplingwire means bridges two discs and couples together the discs on eitherside of the said two bridged discs.

The said second coupling wire means is constructed in such a manner withrespect to length and diameter that it will produce a phase shift of theenergy transferred therethrough, within the passband frequency. Sincethe circle mode discs resonate in phase at the lower edge of thepassband, the energy transferred through the discs will be cancelled bythe energy transferred through the bridging coupling wire. At the upperend of the passband each disc resonates 180 out of phase with theadjacent disc. Since two discs are bridged, each having 180 phase shifttherein, the overall result is a phase shift of 360 through the twobridged discs. Again, the 180 phase shifted energy transferred throughthe bridged coupling wire will cancel the energy transferred through thebridged discs with a resultant attenuation pole at the upper edge ofsaid passband.

in accordance with a modification of the invention the phase-invertingbridging coupling wire is caused to bridge one disc rather tan twodiscs. With only one disc bridged there is produced an attenuation poleonly at he lower edge of the filter passband. Such attenuation pole iscreated at the lower edge of the filter passband. Such attenuation poleis created at the lower edge of the passband since the energytransferred through the coupling wire is phase shifted 180 whereas theenergy transferred through the bridged disc has no phase shift therein.At the upper end of the filter passband the phase shift through thebridged disc is also 180 so that it tends to reinforce the energytransferred through the coupling wires. Thus there is no attenuationpole created at the upper edge of the passband.

The above-mentioned and other objects and features of the invention willbe more fully understood from the following detailed description thereofwhen read in conjunction with the drawings in which:

H6. 1 is a perspective view of a mechanical filter in which aphase-inverting coupling wire is constructed to bridge two @1865;

HQ. 2 is a side view of a portion of FIG. 1 showing in more detail thebridging of two discs by the coupling wire;

FlG. 3 is an equivalent electrical circuit of the mechanical filter ofFIG. ll;

FIGS. 3a and 4a are detailed circuit transformations employed intransforming the equivalent circuit of FIG. 3 into the equivalentcircuit of FIG. 5;

FIG. 5 is the circuit of FIG. 3 after it has been transformed by thetransformation techniques shown in FIGS. 3a and 5;

FIG. 6 is another embodiment of the invention wherein only one disc isbridged by the phase inverting coupling wire to thereby produce anattenuation pole only at the lower end of the filter passband;

MG. 7 is a side view of a portion of the structure of FIG. 6 to moreclearly show the bridging of the single disc by the phaseinvertingcoupling wire;

FIG. 8 is an equivalent electrical circuit of the structure of FIG. s;

FIG. 9 is an equivalent circuit derived from the circuit of FIG. 8 andusing the transformation techniques shown in FIGS. 3a and I;

FIG. III is a plot of the transfer impedance of the longitudinal mode ofa bridging coupling wire as its length is varied, and more specificallyshows those lengths where phase inversion with respect to the shortercoupling wires occurs and where this phase inversion does not occur;

FIG. II is a series of curves showing the transfer impedance of abridging wire for the longitudinal mode and also for the sheer mode asits length is varied, and further shows the composite transfer impedanceof the bridging wire vibrating in a complex combination of both itslongitudinal and its sheer modes of vibration, as the length of thecoupling wire varies;

FIG. I2 is a plot of frequency response of the circuit of FIG. 5; and

FIG. I3 is a similar plot for the circuit of FIG. 9.

This specification is organized in the following manner:

I. STRUCTURE OF FIG. I

A. General Sescription B. Phase Inversion In Coupling Wire C. EquivalentCircuit Of FIG. i and Transformations II. STRUCTURE OF FIG. '6

A. General Description B. Equivalent Circuit Of FIG. 6 andTransformations r. STRUCTURE OF FIG. 1

A. General Description Referring now to FIG. I there is shown a six-discmechanical filter with the discs being identified by referencecharacters 2i), 2i, 22, 23, 24, and 25. Four coupling wires 26, 2'7, 28,and 29 are positioned along the perimeters of the six discs and parallelto the common axis thereof. Such coupling wires 26 through 29 are weldedto the perimeters of the discs and function to hold the discs inposition and also to transfer energy along the filter from disc to disc.

Input means 70 comprises a rod 3i of a magnetrostrictive material suchas a ferrite and which is secured to the end disc 20, and upon which iswound an input winding 32, having an inherent resistence 35 therein andan inherent resonating capacitance 34 thereacross. The input signal issupplied from source 36. The output of the mechanical filter iscontained within the dotted block 7I and includes a ferrite core 37secured firmly to end disc and upon which is wound winding 38. Capacitor39 represents the resonating capacity thereacross. A load resistor Illrepresents, generally, any suitable utilization means for the mechanicalfilter and also includes the inherent resistance of winding 38.

The essence of the present invention is found in the coupling wire 3dand the two bridged discs 22 and 23. The bridged dims 22 and 23 eachhave a flat surface 48 and 4), respectively, formed thereon whichpermits the coupling wire 359 is secured at its two ends to the discs 21and 24. As discussed briefly above, the length of the coupling wire iscritical. It is a characteristic of a rod such as coupling wire 30 thatthe phase or" the energy transferred therethrough at frequencies whosewavelengths lie between a half and a full wavelength of the naturalresonance frequencies of the coupling wire will be l from the phase ofenergy transferred through the coupling wires 26 to 29, which havelengths of less than one-half wavelength. A typical example might be asfollows: At kI-Iz. a coupling wire of the material and a diameteremployed in mechanical filters would have a longitudinal mode wavelengthof almost 2 inches. Thus it would be necessary to have coupling wires ofat least l inch in length to fall within the one-half to a fullwavelength criteria. As a practical matter, a mechanical filterconstructed with a l-inch bridging coupling wire would not be feasiblesince it would require the discs to be spaced too far apart to maintainstructural strength. However, in the range of 400 kHz. or more, thecoupling wire length decreases to about a quarter of an inch or less,which does permit practical spacing between the discs of the filter.

Before considering the equivalent circuit of FIG. 3 and thetransformations following, reference is first made to the curves ofFIGS. III and II which show in more detail the phenomena occurring inthe coupling wire.

B. Phase Inversion In Coupling Wire In FIG. I0 there is shown a plot ofthe transfer impedance of a coupling wire as the length of the rod isvaried. The impedance is plotted along the y-axis and the length alongthe xaxis. More specifically, the x-axis is plotted in terms of wL/ Vwhere w and l/ are substantially constant, and the length L of thecoupling rod varies.

In FIG. 10 only the longitudinal mode of vibration of the rod isconsidered. It can be seen that for a length of the rod less than halfthe acoustical wavelength of the driving signal there is no phaseinversion, as is shown by curve 130. Similarly, for rod lengths greaterthan the acoustical wavelength of the driving signal there is no phaseinversion, as is shown by curve 132. However, for bridging wire lengthswhich lie between one-half of an acoustical wavelength and a fullacoustical wavelength of the bridging coupling wire for the givenfrequency to, phase inversion does occur, as represented by the curveIBI of FIG. It).

In actual practice, however, the longitudinal mode of vibration of thecoupling wire is not only mode present. Also present is a sheer mode ofvibration, of the coupling wires, which is introduced by the circularmode of vibration of the disc which contains a component of motionperpendicular to the axis of the coupling wire, thereby introducing saidsheer mode. In FIG. 11 there is shown not only the independent effectsof the longitudinal mode of vibration and the sheer mode of vibration,but also the superimposed effect of the two modes of vibration. Morespecifically, in FIG. II the longitudinal mode of vibration is shown bythe curves I30, IBI, and I32. The sheer mode of vibration is shown bythe curves I34 and 133. The resultant mode of vibration, which is ineffect the longitudinal and sheer mode of vibrations superimposed oneupon the other, is shown by the curves I37 and I36.

It can be stated generally that in both FIG. I0 and FIG. II a phaseinversion occurs where the curve is above the x-axis and that no phaseinversion occurs where the curve is below the xaxis. Thus in FIG. IIphase inversion occurs between the pair of dotted lines I40 and I41 andthe pair of dotted lines I42 and 143. In the actual design of amechanical filter the area between the dotted lines M0 and 1411 ispreferred to the area between dotted lines I42 and I43 for two reasons.Firstly, the length of the coupling wire for a given frequency isshorter than would be required if the response between dotted lines I42and I43 were utilized. Secondly, the permissible variation of lengthbetween dotted lines and MI is greater than that between dotted linesI42 and M3.

As indicated in FIG. II, the dotted line Mil represents a length of thecoupling wire which is equal to one-half the acoustical wavelength ofsuch coupling wire at the driving frequency. Similarly, it can be seenthat dotted line MI represents a length of coupling wire equal to aboutthreefourths the acoustic wavelength of said coupling wire at thedriving frequency.

C. Equivalent Circuit Of FIG. I

Referring now to FIG. 3 there is shown the equivalent circuit of themechanical structured of FIG. I. In FIG. 3 the tuned circuits 52, 53,S4, 55, 56, and 57 correspond to the discs 20, 21, 22, 23, 24, and ofFIG. I. The inductors L and L and the single inductor L represent thecoupling wires 26, 27, 28, and 29 of FIG. I, and the inductor 60 of FIG.3 represents the coupling wire of FIG. I.

It is to be noted that the inductor 60 is represented as a L,. in FIG.3. Such a negative inductive value is derived from the fact that thecoupling wire 30 inverts the phase of the signal transferredtherethrough.

The input signal source 50 and the resistor 51 of FIG. 3 correspond tothe input signal source 36 and the input resistor of FIG. 1. Similarly,the output resistor 58 of FIG. 3 corresponds to the output resistor ofFIG. 1. Capacitors 34 and 39 of FIG. 1 are shown as C in FIG. 3 and thecoils 32 and 38 are shown as L in FIG. 3. The circuit within dottedblock 49 represents the bridged section of FIG. 1 including resonators21, 22, 23, and 24.

In FIGS. 4 and 4a there is shown a circuit transformation useful intransforming the circuit of FIG. 3 to the form shown in FIG. 5. It canbe seen in FIGS. 4 and 4a that a pi network consisting of inductors 61,62, and 63 can be transformed into an inductor 64 in series with anegative I:1 transformer 65. In order to create a pi network of the typeshown in FIG. 4 from the circuit of FIG. 3, the two inductors 75 and 76of FIG. 3 are each split into two separate inductors consisting of twoinductors in parallel, one identified as L' and the other identified as-L,, as shown in FIG. 3a.

The transformation of FIGS. 4 and 4a can then be incorporated directlyin the circuit of FIG. 3 to produce the resultant equivalent circuitshown in FIG. 5. It is to be noted specifically that the inductor 60 ofFIG. 3 is a negative inductor (-1 since it represents the bridgingcoupling wire which has been constructed to invert the phase of thesignal transferred therethrough.

Thus in FIG. 5, when a second minus sign is added to inductor L,. ofFIG. 3 due to the transformation, the result is that inductor 66 becomespositive.

The general purpose of transforming the circuit FIG. 3 into that of FIG.5 is to provide an equivalent circuit both easy to analyze and tocompute specific component values therefor to produce the desiredfrequency response characteristics. It is possible to transform thecircuit FIG. 5 into other equivalent circuits for purposes of suchcomponent value computation, if desired. For example, in theaforementioned US application Ser. No. 547,947, filed May 5, 1966, byRobert A. Johnson et al. there is shown additional transformations ofthe type circuit shown in FIG. 5 of the instant specification.

The state of the art, however, is such that the values of the componentsof FIG. 5 can be obtained directly therefrom. Expressions for suchcomponent values are given below.

where R is the image impedance at where r= wlw 2yb) Once the values ofthe components of the circuit FIG. 5 are obtained the correspondingvalues of the circuits of FIG. 3 can easily be obtained. It is then amatter of design to fabricateth'e discs and coupling wires of themechanical filter of FIG. 1 to correspond to the electrical componentvalues of FIG. 3. More specifically, the size and shape of the discs,and the spacing therebetween, and the size and spacing of the couplingwires required to create an equivalency with the component values ofFIG. 3 are well known in the art and will not be discussed in detailherein.

The resultant frequency response curve produced by the structure of FIG.1 is shown in FIG. 12 with the two attenuation poles being identified byreference characters and 91.

II. STRUCTURE OF FIG. 6

A. General Description Referring now to FIG. 6 there is shown amodification of the invention in which the bridging coupling wire 109bridges only one disc 112 rather than the two discs of FIG. 1. Theprincipal difference in the operation of the structure of FIG. 6 andthat of FIG. 1 is that an attenuation pole is created only at the lowerend of the passband of FIG. 6, whereas attenuation poles were created atboth the lower and the upper end of the ,passband in the structure ofFIG. 1.

The structure of FIG. 6 is comprised of five discs, 100, 101, 102, 103,and 104 which are held in stacked relation by coupling wires 105, 106,107, and 108 positioned along the perimeters of the disc and weldedthereto as shown in the drawing. The coupling wire 109 is welded to disc101 and 103 but bridges disc 102 over the flat surface 112 formed on theperimeter thereof.

Input means 110 and output means 111 of FIG. 6 correspond to the inputmeans 70 and the output means 71, respectively, of FIG. 1.

In FIG. 7 there is shown a side view of the discs 101, 102, and 103 ofFIG. 6 and the coupling wire 109. In FIG. 7 it can be seen how thecoupling wire 109 bridges the center disc 102 over the flat surface 112formed on the perimeter thereof.

As in the case of the structure of FIG. 1, coupling wire 109 .has alength which lies between one-half and three-fourths of the acousticalwavelength of said coupling wire at the driving frequency, which isnominally the center frequency of the filter passband. Also as in thecase of the structure of FIG. 1,

the structure of FIG. 6. The circuit of FIG. 8 is quite similar to thatshown in FIG. 3 except that the negative inductor 1 22 bridges only onetuned circuit 118 instead of two tuned circuits. Such difference is dueto the fact that coupling wire 109 of FIG. 6, which the inductor 122represents, bridgesonly one disc 102.

Using the transformation of FIGS. 4 and 4a the equivalent circuit ofFIG. 8 is transferred into the equivalent circuit of FIG. 9. Here again,as in the transformation of the circuit of FIG. 3 to FIG. 5, thenegative inductor 122 of FIG. 8 becomes the positive inductor 123 inFIG. 9 since a second minus sign is involved.

In FIGS. 8 and 9 the circuitry within dotted blocks 99 and 99'respectively, represents a three resonator bridged section.

The expression for finding the component values of the equivalentcircuit of FIG. 9 are as follows:

where R istheima ge impedance where g y :l: y 1

and Where y =frequency of infinite attenuation where v= \/1 y and F:

p2: l F 2) rr The resultant frequency response characteristic producedby the structure of FIG. 6 is shown generally in FIG. 13 with theattenuation pole occurring at point 92 at the lower edge of thepassband.

It is to be noted that the forms of the invention shown and describedherein are but preferred embodiments thereof and that various changescan be made in the number and arrangement of coupling wires and theproportionate dimensions of the components of the filter withoutdeparting from the spirit or scope of the invention.

I claim:

1. A mechanical filter constructed to produce an attenuation pole on atleast one side of the filter passband and comprising:

Tpliirality of discs stacked one upon the other along a common axis andspaced apart a distance equal to less than one-half wavelength of thenominal center frequency of said passband; first coupling wire meanspositioned substantially parallel to said common axis and physicallysecured to the perimeters of at least N a ja n me fsai dis swhcreN is e3; and second coupling wire means positioned substantially parallel tosaid common axis and physically secure to the perimeters of the two enddiscs of said N discs and physically separated from the discstherebetween; and said second coupling wire means constructed to have alength to produce a phase shift of in the energy transferredtherethrough within said passband. 2. A mechanical filter in accordancewith claim 1 in which N is an odd integer.

3. A mechanical filter in accordance with claim 1 in which N=3.

4. A mechanical filter in accordance with claim 1 in which N is an eveninteger.

5. A mechanical filter in accordance with claim 1 in which N=4.

6. A mechanical filter means constructed to produce anati tenuation poleon at least one side of the filter passband and comprisin a plur ity ofN circle mode discs stacked one upon the other along a common axis,where N 2 3; first coupling wire means positioned longitudinally alongsaid stack of discs and physically secured tot he perimeters of saiddiscs; and second coupling wire means positioned longitudinally alongsaid stack of discs and physically secured only to the end discs of saidN discs to effect a bridging of the discs positioned between said enddiscs; and said second coupling wire means having a length substantiallyin the range extending from 9%.! to A where his the acousticalwavelength of the natural resonant frequency of the wire material for agiven frequency f in the filter passband, and producing 180 phase shiftof energy in said passband. 7. A mechanical filter in accordance withclaim 6 in which N=3.

8. A mechanical filter in accordance with claim 6 in which N is an oddinteger.

9. A mechanical filter in accordance with claim 6 in which N is an eveninteger.

10. A mechanical filter in accordance with claim 6 in which N=4.

1. A mechanical filter constructed to produce an attenuation pole on atleast one side of the filter passband and comprising: a plurality ofdiscs stacked one upon the other along a common axis and spaced apart adistance equal to less than one-half wavelength of the nominal centerfrequency of said passband; first coupling wire means positionedsubstantially parallel to said common axis and physically secured to theperimeters of at least N adjacent ones of said discs, where N is 3; andsecond coupling wire means positioned substantially parallel to saidcommon axis and physically secure to the perimeters of the two end discsof said N discs and physically separated from the discs therebetween;and said second coupling wire means constructed to have a length toproduce a phase shift of 180* in the energy transferred therethroughwithin said passband.
 2. A mechanical filter in accordance with claim 1in which N is an odd integer.
 3. A mechanical filter in accordance withclaim 1 in which N
 3. 4. A mechanical filter in accordance with claim 1in which N is an even integer.
 5. A mechanical filter in accordance withclaim 1 in which N4.
 6. A mechanical filter means constructed to producean attenuation pole on at least one side of the filter passband andcomprising: a plurality of N circle mode discs stacked one upon theother along a common axis, where N 3; first coupling wire meanspositioned longitudinally along said stack of discs and physicallysecured tot he perimeters of said discs; and second coupling wire meanspositioned longitudinally along said stack of discs and physicallysecured only to the end discs of said N discs to effect a bridging ofthe discs positioned between said end discs; and said second couplingwire means having a length substantially in the range extending from 1/2lambda to lambda , where lambda is the acoustical wavelength of thenatural resonant frequency of the wire material for a given frequency f1in the filter passband, and producing 180* phase shift of energy in saidpassband.
 7. A mechaniCal filter in accordance with claim 6 in which N3.
 8. A mechanical filter in accordance with claim 6 in which N is anodd integer.
 9. A mechanical filter in accordance with claim 6 in whichN is an even integer.
 10. A mechanical filter in accordance with claim 6in which N 4.